The present invention provides synthetic processes for shorter and more efficient syntheses of functionalized carbocyclic (polyol) compounds and substituted sugar analogs including novel classes of cyclopentene and cyclohexene polyol intermediates and derivatives thereof.
Fructose 2,6-bisphosphate (FIG. 1, compound 1) is formed by phosphorylation of fructose 6-phosphate, a key substrate in the glycolysis pathway, in a reaction catalyzed by phosphofructokinases (PFK""s) (Hasemann et al. Structure, 4:1017, 1996; Pilkis et al., Ann. Rev. Biochem. 64:799, 1995; and Rousseau et al., Nucl. Acid Res. Mol. Biol. 45:99, 1993). Fructose 2,6-bisphosphate is a key regulatory molecule for glycolysis and gluconeogenesis via its potent stimulatory effect on phosphofructokinase-1 activity and its inhibitory effect on fructose 1,6-bisphosphatase. The importance of this regulatory mechanism underscores the need for analogs of fructose 2,6-bisphosphate with which to probe and potentially control mechanisms that govern anaerobic glycolysis. To date, such controlled modulation of glycolysis has not been accomplished. In this regard, certain carbocyclic sugar analogs might be particularly useful. Accordingly, there is a need to generate carbocyclic sugar analogs efficiently, economically and in good yield.
Ring closing metathesis (RCM) has recently become a powerful tool for the synthesis of medium (5-8) to large (10-13 and higher) carbo or heterocycles (Furstner and Langemann, Synthesis 792, 1997; Grubbs and Miller, Acc. Chem. Res. 28: 446, 1995; Nicaloau et al., J. Am. Chem. Soc. 119:10073, 1997; Crimmins and Choy, J. Org. Chem. 62:7548, 1997; Schmalz, Angew. Chem. Int. Ed. Engl. 34:1833, 1995; and Arisawa et al., Syn. Lett. 1179, 1997). More recently there have been two reports dealing with RCM on functionalized substrates. The first is the synthesis of the six-membered poylsubstituted cyclohexene valiolamine employing Schrock""s catalyst (Sellier et al., Tetrahedron Lett. 40:853, 1999) and the second is the synthesis of the seven-membered heterocyclic oxepine skeleton (Ovaa et al., Tetrahedron Lett. 39:3025, 1998) utilizing Grubbs"" catalyst.
Carbocyclic D-fructofuranoside (FIG. 1, compound 2) has been synthesized in twelve-steps with cyclopentane ring closure achieved by free radical-mediated cyclization (Wilcox and Guadino, J. Am. Chem. Soc. 108:3102, 1986). This synthetic approach is difficult, provides low yields and is not useful for commercial synthesis. There is a need in the art to improve the synthesis of carbocyclic polyols and substituted analogs because such agents may be active in affecting intermediary metabolism (glycolysis).
By controlling glycolysis, it may be possible to interfere with some diseases that manifest aberrant activity or flux in glycolytic pathways. Such diseases would include, for instance, insulin-dependent and non-insulin dependent diabetes mellitus (Nathan, Ann. Int. Med. 124:86, 1996; and Nishimura et al., J. Biol. Chem. 269:26100, 1994) and cancer (Chesney et al., Proc. Natl. Acad. Sci. USA 6:3046, 1999; Argiles et al., J. Mol. Cell. Biochem. 81:3, 1988; and Hue et al., Adv. Enz. Regul. 33:97, 1993). Unlike native carbohydrates such as phosphorylated fructose, carbocyclic sugar analogs cannot be metabolized and may therefore have prolonged effects on the control of glycolysis, offering the opportunity to interfere with or modulate both normal and pathological mechanisms that regulate glycolysis.
The present invention provides an improved process for synthesis of carbocyclic sugar analogs by annulation of an olefinic intermediate. One embodiment of the inventive process provides an eight-step synthesis procedure that is a significant improvement over a twelve-step process published in 1986. Specifically, one embodiment of the invention provides a process (according to Scheme I) for synthesizing a class of desired intermediate products for further derivatization to provide final carbocyclic sugar analogs, comprising the steps of:
(a) subjecting a pentose or a hexafuranose precursor compound that is not protected at the anomeric hydroxyl group to a Wittig reaction to effect ring opening and generate an alkene intermediate product;
(b) oxidizing the alkene intermediate product of step (a) to yield a keto-alkene intermediate product;
(c) effecting the nucleophilic addition of vinyl carbanion to the keto-alkene intermediate product of step (b) under Grignard conditions to provide a 1,6-heptadiene product;
(d) optionally protecting the generated free alcohol of the 1,6-heptadiene product of step (c) to generate a fully protected 1,6-heptadiene product;
(e) subjecting the 1,6-heptadiene product of step (c) or step (d) to ring closure metathesis (RCM) conditions with an RCM catalyst to effect ring closure and generate a functionalized cyclopentene product.
This synthetic scheme is (Scheme I) summarized in FIG. 2. The novel functionalized cyclopentene product of step (e) may then be deprotected and/or further functionalized or elaborated to provide carbocyclic sugar analog agents to probe, characterize and modulate glycolysis and the normal and pathological mechanisms that operate to control glycolysis in healthy and diseased individuals. For example, the functionalized cyclopentene product of step (e) is subjected to Sharpless epoxidation to provide a corresponding anhydro cyclopentane product advantageous for the generation of carbocyclic sugar analogs (FIG. 3).
Alternatively, at step (a), a series of reactions including sequential oxidation, methyl Grignard addition and acetylation followed by thionyl chloride elimination, optionally executed as a one-pot reaction, (according to Scheme II) may be substituted for the Wittig reaction to effect ring opening and generate an alkene intermediate product (see FIG. 4).
In a second alternative, a hexopyranose precursor compound is instead utilized at step (a) of Scheme I, and after step (b), the keto-alkene intermediate product is subjected to a Wittig reaction to provide a 1,6 heptadiene product that is suitable for step (e).
In a further embodiment of the general method (see FIG. 5), a hexopyranose precursor compound is utilized instead at step (a) of Scheme I, to provide a 1,7 octadiene intermediate product at step (c), such that the process provides a corresponding functionalized cyclohexene product (according to Scheme III, FIG. 5):
(a) subjecting a hexopyranose precursor compound (14) that is not protected at the anomeric hydroxyl group to a Wittig reaction to effect ring opening and generate an alkene intermediate product (15);
(b) oxidizing the alkene intermediate product of step (a), 15, to yield a keto-alkene intermediate product (16);
(c) effecting the nucleophilic addition of vinyl carbanion to the keto-alkene intermediate product of step (b), 16, under Grignard conditions to provide a 1,7-octadiene product (17);
(d) optionally protecting the generated free alcohol of the 1.7-octadiene product of step
(c) to generate a fully protected 1,7-octadiene product (18);
(e) subjecting the 1,7-octadiene product of step (c), 17, or step (d), 18, to ring closure metathesis (RCM) conditions with an RCM catalyst to effect ring closure and generate a functionalized cyclohexene product (19).
Any of the functionalized cyclopentene or cyclohexene products of step (e), from Scheme I, II, or III, are converted into corresponding carbacyclic sugar analogs. In some cases, this is accomplished by carbonylation (hydroformylation) followed by reduction, as described in detail in Example 9 and Scheme V with completion in Scheme VI, a process not previously demonstrated on highly functionalized polyol (i.e., polyhydroxylated) systems. Alternatively, the starting materials can be halo-sugars or azo-sugars. Preferably, any of the RCM catalysts above are selected from the group consisting of a Grubbs"" catalyst and a Schrock""s catalyst. More preferably, the protecting groups are benzyl or alkylsilyl groups, and combinations thereof. Still more preferably, the pentose precursor compound is 2,3,5-tri-O-benzyl-D-arabinofuranose.